The invention is directed to a process for the resolution of the racemate S-(carboxymethyl)-(RS)-cysteine, especially for the purpose of recovery of S-(carboxymethyl)-(R)-cysteine. This substance is needed for pharmaceutical purposes and serves for example, as a mucolyticum.
It is known to produce S-(carboxymethyl)-(R)-cysteine by reacting (R)-cysteine--also called L-cysteine--with chloracetic acid in alkali medium (Armstrong, J. Org. Chem. Vol. 16(1951) pages 749 to 753).
The (R)-cysteine needed for this purpose as starting material is generally obtained from keratin containing natural materials. For this purpose these are hydrolyzed; the (RR)-cystine set free is separated and reduced to (R)-cysteine (Org. Synth. Vol. 5(1925) pages 39 to 41); German OS No. 2653332 (and related Scherberich U.S. Pat. No. 4,245,177 the entire disclosure of which is incorporated by reference and relied upon), Vigneaud, J. Amer. Chem. Soc. Vol. 52(1930) pages 4500-4504). However, suitable natural materials are only available to a limited extent.
In the synthetic production of cysteine, for example, from thiazolines-3 substituted in the 2-position via the corresponding thiazolidin-4-carbonitriles the racemate (RS)-cysteine is formed. The entire disclosure of German OS 2645748 is hereby incorporated by reference and relied upon. It is known to obtain (R)-cysteine by reacting the (RS)-cysteine with dicyandiamide to form (RS)-2-guanidine-1,3-thiazolidin-4-carboxylic acid, from this with the help of the copper complex salt of (R)-aspartic acid there is separated the (R)-2-guanidine-1,3-thiazolidin-4-carboxylic acid and subsequently there is split off from this the (R)-cysteine (German AS No. 1795021). This process is for the recovery of the (R)-cysteine thus is cumbersome and expensive, which is unsuited for use on an industrial scale.